1. Field of the Invention
The present invention is directed to a gradient coil system suitable for use in a nuclear magnetic resonance tomography apparatus.
2. Description of the Prior Art
Gradient coil systems are required in nuclear magnetic resonance tomography devices for generating gradient fields in the x-direction (G.sub.x) and in the y-direction (G.sub.y). It is known to provide a hollow cylindrical carrier having a cylinder axis proceeding in the z-direction of a rectangular x-y-z coordinate system, with the origin of the coordinates disposed in the center of the imaging region. A fundamental magnetic field B.sub.z is also generated, which is oriented in the z-direction. It is known to provide a set of 4 coils for generating the G.sub.x gradient and a separate set of saddle coils for G.sub.y gradient. Each saddle coil has straight conductor sections, connecting conductor arcs, with the straight conductor sections being oriented along the z-direction so that each saddle coil generates a gradient field in the same volume as the fundamental magnetic field. The straight conductor sections may be arranged in the winding bed of a carrier.
In nuclear magnetic resonance devices of this type, it is known that the fundamental magnetic field aligns the nuclear spins in an examination subject, such as a human body, and an RF system is provided for generating an RF field to excite nuclear spins in the examination subject, and to receive the resulting nuclear magnetic resonance signals. Gradient coils which proceed in the direction of the fundamental field generate a linearly changing magnetic field in this direction, and are required for the slice selection and for the spatial allocation of the signals in the slice. Further gradient coils similarly generate a magnetic field proceeding in the direction of the fundamental field but which changes in two directions perpendicular thereto. By selective excitation of these gradient coils, the phase of the RF signal can be influenced, so that an image of the slice plane of the examination subject can be derived based on the nuclear spin distribution.
A known nuclear magnetic resonance tomography device includes a system of gradient coils which simulate a hollow cylinder having a radius R, and having a cylinder axis proceeding in the z-direction of a rectangular x-y-z coordinate system having the coordinate origin in the center of the imaging region. The fundamental magnetic field B.sub.z also is oriented in this direction. At least two annular individual coils arranged symmetrically relative to the x-y plane, and oppositely traversed by current, are provided for generating a field gradient G.sub.z, in the same direction as the fundamental field which is approximately constant in the imaging region. In this known system, further gradient coils are provided for generating the y-gradient G.sub.y and the x-gradient G.sub.x. Respective sets of coils, each consisting of a set of 4 saddle-shaped coils, are provided for respectively generating these gradients. Each pair is respectively arranged symmetrically relative to the x-y plane, which is also the imaging plane of the tomogram to be produced. These saddle coils generate a field gradient in the x-direction ##EQU1## which is substantially constant in the imaging region, and generate a corresponding field gradient in the y-direction ##EQU2## The saddle coils each contain straight conductor sections proceeding in the z-direction, and contain conductor arcs parallel to the x-y plane which extend around the circumference of an imaginary cylinder. The directions of current conduction in the adjacent straight conductor sections of the two individual coils of each coil pair are the same. The directions of the current, however, are opposite in the conductor sections arranged symmetrically relative to the x-y plane. The conductor arcs of the saddle coils facing toward the x-y plane are each divided so that a further conductor arc arises. The conductor arcs are arranged at predetermined distances from the x-y plane and the coupling between the conductor arcs increases with increasing distance from this plane. Such an arrangement is described in U.S. Pat. No. 4,486,711.
Given the use gradient coils in a nuclear magnetic resonance tomography apparatus for fast pulse sequences, the share of the higher audio frequencies from about 1 kHz through 10 kHz which is contained in the spectrum of the pulse sequences increases. To avoid both an increased power consumption and field distortions in this range due to the skin effect in solid conductors, stranded or cable conductors are used for the windings of the gradient coils. The individual saddles of a coil set of the gradient coils for a predetermined spatial coordinate must be connected to one another along short paths exhibiting low inductance. Because cable conductors contain individual wires insulated from each other, a solid block of solder, which can cause disturbing eddy currents, must be used to provide an electrical connection at the conductor ends.
Another known gradient coil system for use in a nuclear magnetic resonance tomography apparatus is described in European application 0 274 149. This known gradient coil system is arranged on a hollow cylindrical carrier having a cylinder axis proceeding in the z-direction of a rectangular coordinate system having the coordinate origin in the center of the imaging region. The fundamental magnetic field is also oriented in the z-direction. A coil set consisting of four saddle coils is provided for generating the x-gradient, and another set of four saddle coils is provided for generating the y-gradient. Each saddle coil consists of three sub-coils which are arranged on the same cylindrical surface. The conductor arc of the outer sub-coil and the conductor arc of the middle sub-coil proceed close to the symmetry plane z=0, whereas the conductor arc of the inner sub-coil is farther from this plane. One conductor arc of the middle sub-coil contains a concave curvature. This coil system can thus not be wound without substantial outlay.